1. Field of the Invention
The present invention relates to a photometer to receive light from a subject to obtain exposure for an auto exposure camera or more particularly to a camera photometer in which metering light equivalent to photographic light can be incident to a light receiving element of the photometer.
2. Description of the Related Art
Cameras equipped with an auto focusing device or an auto exposure device are now widely used so that even a beginner in photography can readily take wonderful photos without missing a good timing to press the shutter button. An auto focusing device is a device to measure a distance to a subject by a predetermined operation such as aiming the camera to the subject to press the shutter button just a half way, and to focus on a subject automatically.
The auto exposure device is a device to measure the brightness of a subject as the shutter button is pressed to take a picture and automatically determines the exposure. Light beams required to measure the brightness of a subject are incident, with an image formed by a condenser lens, to a light receiving element or detector, such as a CdS element, to measure the brightness of the subject. To bring such measuring light beams incident to a light receiving element, a light shield mask or a diaphragm is arranged between a condenser lens and a light receiving element, as described in Japanese Utility Model Registration No. 2515036, in order to prevent light beams unnecessary for measurement from being incident to the light receiving element. Then, Japanese Laid-Open Patent Application No. 1999-2853 describes that a light shield layer is arranged inside a light receptor window to limit the range of light transmission to a light receptor window and also that an incident light limiting member equipped with a light receptor diaphragm between a light receptor window and a light receptor for a measuring element. These elements are arranged to limit incidence of light beams unnecessary for measurement, limiting the range of incidence with a light receptor diaphragm, while the incident light limiting member is black in its inside to prevent irregular reflection inside the member.
To take a photograph of a subject, exposure is given by light transmitted through a photographic lens. However, when a photographic lens and a light receiving element are distant from each other, spatial parallax may occur between photographic light for exposure and metering light incident to the light receiving element. To make the photographic light and the metering light as equivalent to each other as possible, the light receiving element should preferably be placed in the vicinity of the photographic lens.
For better photographing enjoyment, a camera is frequently equipped with a variable focusing device, such as a zoom lens device and/or a focal distance switching device, which is capable of changing the focal distance of a photographic lens. The lens barrel of the photographic lens travels forward or backward in the direction of the optical axis, when switching the focal distance with such a variable focusing device. With new cameras, the focal distance is changed at a larger and larger rate, pushing the lens barrel more and more forward. As a result, the lens barrel has a wider area of side surface to reflect more outside light beams. A wide use type of camera, painted on the outer surface around the lens barrel to give metallic luster like an expensive camera, particularly reflects more light beams. Moreover, even more light beams are reflected when nearly back lighted.
Light beams, reflected on the outer surface around the lens barrel, also impinge upon the light receiving element, trying to be incident into the light receptor through the light receiving window arranged before the light receiving element, but are intercepted e.g. by the light shield layer. But when more light beams are reflected on a wider area of outer surface around the lens barrel, more light beams try to impinge upon the light receptor and the light shield layer may fail to prevent all of them from penetrating it. Moreover, when the light receiving element is placed in the vicinity of the lens barrel to make the metering light equivalent to the photographic light, reflected light beams reach there without being attenuated, inviting penetration of more intensified light. Then, some reflected light beams, incident at an end of the light receiving window, may penetrate into the light receptor. Such penetrating light may be irregularly reflected inside the light receptor to reach the light receiving element at last. These effects can cause erroneous metering, resulting in improperly exposed photographs.
An object of the present invention is to provide a camera photometer capable of assuring incidence of metering light to a light receiving element in spite of increased or intensified incident light beams.
The above object is technically achieved by the camera photometer of the present invention comprising a photo metering window arranged on the front face of a camera to make incidence of subject light there from, a condenser lens to collect the light to make it incident to a light receiving element, to control exposure based on subject brightness information of light beams received at the light receiving element, characterized in that a light shield mask having an opening behind the photo metering window is arranged to form a wide range light shield, at a part of the light shield mask, shaped to cut off light over a wider range than the rest.
Subject light and other outside light beams are incident from the photo metering window. Light beams for photometry incident from subject light are incident to a condenser lens to form an image on the light receiving element for measurement of subject brightness. Outside light beams unnecessary for photometry are intercepted by the light shield mask and prevented from being incident on the light receiving element. In the presence of high luminous energy light among the outside light beams, the light shield mask is set to place the wide range light shield where the light beams are incident, thereby intercepting and preventing the high luminous energy light from penetrating into the light receptor. Only light beams necessary for measuring subject brightness can be incident at a maximum to the light receiving element.
The camera photometer of the present invention is characterized in that the edge of the opening of the light shield mask is formed to be continuous between a part along a circular arc and a part along a chord of the circular arc and to form a wide range light shield to intercept light beams over a wider range at the part facing the chord.
A condenser lens is circularly formed in its outer shape in general and unnecessary outside light may be included among light beams transmitting around there. The light shield mask is formed to have a circular opening having an inner diameter smaller, as needed, than the outer diameter of the condenser lens in order to intercept light beams transmitted around the condenser lens. This circle may be partly linked not with a chord but with a circular arc to form an opening e.g. into a circular shape partly missing a circular arc. However, a light shield mask can be easily made, if the edge of the circle is partly linked with a chord of the circle to form the wide range light shield at a part of the circle.
The camera photometer of the present invention is characterized in that the wide range light shield is placed at a part facing the lens barrel of the photographic lens.
For example, the lens barrel is extended forwardly for the photographic lens to have a longer focal distance. Then, a larger outside area around the lens barrel is exposed to have a maximum area when extended all the way to the front end at a maximum focal distance, also to have a maximum light reflection on the outer surface around the lens barrel. Incidence of this high luminous energy reflected light into the light receptor comprising a light receiving element and condenser lens is minimized by placing the wide range light shield to face the lens barrel.
The camera photometer of the present invention is characterized in that the optical axis of the light receptor comprising the light receiving element, condenser lens, photo metering window etc. is arranged in the vicinity of the outer surface of the lens barrel of the photographic lens.
High precision photo metering data can be obtained, efficiently taking subject light while intercepting reflected light beams from the outer surface around the lens barrel, also with a camera designed to have a photo metering window at its upper front, having a condenser lens and a light receiving element behind it, if the camera has the wide range light shield placed to face the lens barrel of the photographic lens. However, the wide range light shield is more efficiently used, if the light receptor, placed in the vicinity of the lens barrel, has the wide range light shield placed to face the lens barrel, because light beams reflected on the outer surface around the lens barrel to be incident to the light receptor placed in the vicinity of the lens barrel are not so much attenuated as light beams incident to a light receptor distant from there, inviting incidence of more intensified light.
The camera photometer of the present invention is characterized in that the light shield mask is elastic and coupled to the incidence side of the condenser lens.
The light shield mask can be arranged in a more simplified construction by applying it to a condenser lens. In addition, an elastic light shield mask flexibly fits the surface of the condenser lens in close contact with the condenser lens, thereby minimizing incidence of light beams at an end of the condenser lens and assuring prevention of light beams from being reflected at the end.
These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings.